The present invention relates to an optical element having a form birefringence effect generated by a periodic structure having a smaller period than that of a use wavelength (wavelength of incident light), moreover, to an optical apparatus such as an image projection apparatus using the optical element.
An optical element having a so-called “form birefringence effect” generated by a minute periodic structure having a smaller period than that of a use wavelength and being formed on a base member has been proposed in Born & Wolf “Principles of Optics” pp. 705-708. The optical element having the form birefringence effect is often used as a phase plate such as a quarter-wave plate and a half-wave plate (see Japanese Patent Laid-Open Nos. 2003-90916 and 2003-207636).
A phase plate disclosed in Japanese Patent Laid-Open Nos. 2003-90916 and 2003-207636 includes a one-dimensional grating portion having a specific period on the base member.
Since the period of the one-dimensional grating portion is smaller than that of the use wavelength, the one-dimensional grating portion functions as a medium having birefringence without generating diffracted light. When the product of a refractive index difference generated by the form birefringence effect and the height of the one-dimensional grating portion is a half of the use wavelength or a quarter thereof, the one-dimensional grating portion functions as a phase plate.
The one-dimensional grating portion disclosed in Japanese Patent Laid-Open No. 2003-90916 is constituted by two types of media different in wavelength dispersion of refractive index. The phase plate disclosed in Japanese Patent Laid-Open No. 2003-207636 employs air as one of the two types of media.
Such phase plates are used for an image projection apparatus such as a projector. More specifically, for example, specified color light separated from white light emitted from a light source is introduced as linearly-polarized light (e.g., S-polarized light) having a specified polarization direction by a polarization beam splitter. The light reflected by or transmitted through the polarization beam splitter finally enters a quarter-wave plate
The quarter-wave plate converts the entered linearly-polarized light into circularly-polarized light. The circularly-polarized light is then introduced to a reflective-liquid crystal panel. Furthermore, the quarter-wave plate converts the light reflected and image-modulated by the reflective-liquid crystal panel into P-polarized light. The P-polarized light is transmitted through or reflected by the polarization beam splitter to be projected on a screen by a projection lens.
As described above, in the image projection apparatus, the quarter-wave plate gives a significant optical effect to the light projected on the screen. Thus, the quarter-wave plate requires a high transmittance characteristic (low reflectance characteristic) as well as its own functions. When the reflective index of the quarter-wave plate is not low enough, reflections of light between the quarter-wave plate and the reflective-liquid crystal panel are repeated, which forms a ghost image. The ghost image deteriorates the contrast of a projected image. On the other hand, when the transmittance characteristic of the quarter-wave plate is low, the projected image becomes dark.
Japanese Patent Laid-Open Nos. 2003-90916 and 2003-207636, however, do not disclose a transmittance characteristic of the phase plate.
In contrast, phase plates having an improved transmittance characteristic has been disclosed in Japanese Patent Laid-Open No. 2004-139001 and in H. Kikuta et al “Apply. Opt. Vol. 36, No. 7” pp. 1566-1572 (1997).
A one-dimensional grating portion disclosed in Japanese Patent Laid-Open No. 2004-139001 has a two-layer structure formed of materials different in refractive index and dispersion characteristic. Thereby, the phase difference characteristic of the one-dimensional grating portion is improved. Furthermore, materials of the grating portion and the base member are selected so as to reduce a refractive index difference at an interface between the one-dimensional grating portion and the base member to lower the reflectance of the phase plate.
On the other hand, in a phase plate disclosed in H. kikuta et al “Apply. Opt. Vol. 36, No. 7” pp. 1566-1572 (1997), a one-dimensional grating portion is sandwiched by layers of a specified material, thereby reducing reflection generated at interfaces between the one-dimensional grating portion and the specified material layers.
In a phase plate disclosed in Japanese Patent Laid-Open No. 2004-139001, the phase plate has a high transmittance characteristic equal to or more than 95%. However, since only limited materials are available to form the one-dimensional grating portion, it is difficult to further lower its reflectance.
In the phase plate disclosed in H. Kikuta et al “Apply. Opt. Vol. 36, No. 7” pp. 1566-1572 (1997), the reflection generated at the interfaces of the grating portion is reduced. However, an anti-reflection coating on the specified material layer is needed to reduce reflection generated at an interface between the specified material layer and air. Although no method for manufacturing the phase plate is proposed in H. Kikuta et al “Apply. Opt. Vol. 36, No. 7” pp. 1566-1572 (1997), it is expected to be very difficult.